Rotating flameholder



v- 0, 1959 A. w. BLACKMAN, JR.. ETAL ROTATING FLAMEHOLDER Filed June 18, 1956 INVENTORS ARTHUR m BLACKMAN JR. GEORGE o. LEW/S ATTORNEY United States Patent ROTATING FLAMEHOLDER Arthur W. Blackman, Jr., and George D. Lewis, Manchester, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application June 18, 1956, Serial No. 592,207

9 Claims. (Cl. 6039.72)

readily apparent from the following detailed description:

of the drawing in which:

Fig. 1 is a schematic illustration of the pressure pattern in a combustion chamber oscillating in a transverse mode.

Fig. 2 is a partial cross section and a partially schematic illustration of a combustion chamber having the rotating flameholders of this invention, and

Fig. 3 is a schematic illustration of the pressure pattern and positions of the specific flameholders of this invention.

High frequency oscillatory combustion has been experienced in many high output combustors such as rockets, ramjets, and turbojet afterburners. The frequency of these oscillations is usually determined by some natural acoustical frequency of oscillation of the combustion chamber cavity. In most cases where these oscillations occur, the amplitude is suflicient to cause failure of the combustion chamber structural components after a few minutes operation. However, if the amplitude of the oscillations can be controlled, the space requirements of combustion can be reduced by allowing this type of combustion to occur. Because both advantages and disadvantages exist with oscillatory combustion, control is extremely desirable.

The main criterion for sustaining oscillatory combustion appears to be that variations in the combustor heat release rate must occur nearly in phase with the variation in pressure. It can also be shown that the amplitude of the oscillations is a function of this phase relationship. Hence, it should be possible to control the amplitude of oscillatory combustion by artificially adjusting the phase relationships between the heat release and the pressure.

One convenient means of obtaining the desired phase relationship is through the use of a rotating flameholder as'shown in Fig. 1. The flameholder 10 is located in a combustion chamber 12 and is rotated on an arm 14 rotatable about pivot 16. The pressure at point A is a maximum at the instant the flameholder is releasing heat at point A. As the pressure wave recedes from point A toward point B, it is accompanied by the flameholder and both arrive at point B simultaneously to reinforce the pressure oscillation by the addition of heat again at the pressure maximum. It is evident that, to meet the phase requirements, the flameholder in Fig. 1 must be driven (by an external device not shown) at a rotational speed I as 2,911,791 Patented Nov. 10, 1959 ICE equal to the natural acoustic oscillation frequency of the combustion chamber cavity. However, a single flameholder as shown in Fig. 1 is not practical because the high rotationalspeed necessary to synchronize the flameholder with the natural frequency of the combustor would: (1) produce excessive physical stresses in the flameholder and (2) result in excessively high gas velocities relative to he flameholder. Therefore, an alternate means of adding heat in phase with thepressure without excessive flameholder rotational speeds in shown in Figs. 2 and 3.

As seen in Figs. 2 and 3, flameholders 20, 22, 24 and 26 are mounted on rotating arms 28 and 30, respectively, in duct 32. The flameholders 20, 22, 24 and 26 have a V shape with the apex of the V generally pointed upstream. 7 The spanwise dimension of the flameholders are placed at an angle relative to the plane of rotation of the flameholders. The angle at which the flameholders are placed is much the same as that of a propeller blade rotating in a moving stream.

The flameholders 20, 22, 24 and 26 are rotated on their arms 28 and 34) in a combustion chamber defined by the duct 32 which terminates in an aft nozzle 36. A spark gap 38 may be provided immediately downstream of the flameholders. As shown in Fig. 2, an afterburner arrangement is illustrated with fuel nozzles 40 ejecting fuel downstream of the turbine 42. q

The arms 28 and 30 are driven by shafts 44 and 46, respectively, which are supported in bearings 48 and 50. Suitable bevel gears 52 and 54 are provided and are driven from a DC. motor schematically shown at 58. The DC motor may be driven by a suitable source of power such as a battery 60 and the system may include a variable resistance 62 for adjusting the speed of rotation of the motor. The particular rotational speed at which the flameholders are desired to be driven may be defined as follows. However, it should be stated that the flameholders are geared together to maintain a constant predetermined phase relationship therebetween.

The rotational speed and relative positions of the flameholders are adjusted so the position of a flameholder and, hence, the heat release will be in phase with the maximum pressure amplitude which occurs in the duct for a particular acoustical mode of oscillation. Operation of the device is illustrated in Fig. 3 for excitation of the fundamental transverse mode of acoustical oscillation. This mode of oscillation was selected because it is the mode most prone to oscillation when heat release occurs near the wall. The rotating flameholders make /4' of the revolution per cycle of pressure oscillation, and the r.p.rn. of the wheels can be written for the first transverse mode of oscillation as where '11 is the natural frequency of the first transverse mode of gas oscillation. This frequency can be expressed 1.840 V- 1rD where C=velocity of sound in the combustor, ft./sec.,

D=combustor diameter, ft.

For a 40-in. diameter afterburner with a frequency of oscillation of 500 c.p.s., the r.p.m. of the flameholders would be approximately 7500, which is not unreasonable. Properly designed flameholders rotating at this speed should hold flame until axial gas velocities of about 475 ft./sec. are reached. The flameholders are mounted at a predetermined angle or on a pivoting mechanism so that the velocity of the gas relative to the flameholder is perpendicular to the edges of the flameholder The rotating flameholder near the topof the sketch (Fig. 3) lags the bottom flameholder by 45 degrees. This phase relationship between fiameholders will promote screech. Similarly a O-degree phase relationship between the flameholders would prevent screech, and other phase relationships between and-45 degrees would al} low the combustor to screech at pressure amplitudes less than the maximum;

The letters in Fig. 3 indicate successive positions of the flameholders for one complete flameholder revolution which corresponds to four cycles of pressure variation. The lines indicate the variation of acoustical pressure which exists with the fiameholders20, 26 in position a. The diametrically opposite flameholders 22, 24 are in the position e while each flameholder can assume any one of positions a through h. Variations in the weight of thelines indicate qualitative pressure variations with heavy lines indicating large deviations from the average pressure. The plus or minus sign indicates whether the pressure at pointA in Fig. 3 is positive or negative. It can be seen that the over-all effect of the phase relationships shown in Fig. 3 is to cause most of the heat to be released when the pressure is at a maximum, which fulfills the criterion for driving oscillations. Phase relation'-- ships similar to the ones illustrated in Fig. 3 can be constructed to drive other acoustical modes of oscillation.

Various means of-rotating the flameholders are possible, such as dilferent types of electric motors, turbine motors, or a geared drive from a turbojet engine. The only' requirementsfor the flameholder rotating mechanism are (l) the rotational speed must be a proper function of the natural acoustic frequency of the combustor as described above and (2) the proper phase relationship'must be maintained.

By the use of rotating flameholders with the proper phase relationship it is possible to eliminate combustion oscillations or to produce combustion oscillations at will and to control the" amplitude of the oscillations. Thus; as a'ir'sult ofthis invention the size of the required flame holder structure and, hence, the flameholder drag is substantially reduced; As-a result of this inventiona simple yet highly effective flameholder mechanism is providedwhich can in addition control or eliminate high frequency pressure oscillationsin combustion chambers. Furthermore, as a result ofthis invention flameholders are provided which rotate at a speed which is a function of the natural frequency of the combustion chamber to control the phase relationships. between. a varying heat' release and the. pressure maxima' and minima.

Although only one embodiment of this invention has been illustrated and described herein, it will be apparent that various changes and modifications can be made in the construction and arrangement of the various parts without departing, from the scope' of this novel concept.

What it is desired by Letters Patent is:

1. In a combustion chamber having a duct, means for feeding combustible products to said duct for burning therein, and means for igniting said products including flameholder mechanism, said flameholder mechanismcomprising a' pair of arms each rotatable about a diflerentr axis running parallel to the axis of flow through said duct, said axes of rotation being eccentrically located in said duct, wake producing vanes mounted on the extremities of said arms, and means for regulating the speed of rotation of said arms.

2. In a combustion chamber having a duct, means for feeding combustible products to said duct for burning therein, and means for igniting said products including flameholder mechanism, said flameholder mechanism comprising at least two arms rotatable in a plane transversely of the axis of flow through said duct, means for. rotating said arms about a point intermediate the endsthereof, each of said arms being rotated about different axes, and means for regulating the rotation ofsaidarms.

3. In a duct forming a combustion chamber, means for feeding products of combustion to said combustion cross sections, said arms being rotated about separate axes which are substantially parallel to the fluid flow through-said combustion chamber, wake producing ele- IYR'GIHS' mounted and carriedby the extremities. of said arms to-form rotata-b'le-flameholder elements, said elements having effective cross sections relatively larger than said elements, said elements having a length relatively smaller than the length. of said arm, and means for synchronizing the relative rotative angular position of said arms with respect to each other.

4. In a duct forming an elongated combustion chamber,.means for feeding products of combustion to said combustion chamber for burning therein, an exhaust nozzledownstream from said combustionchamber and receivingth'e' exhaust fluids therefrom, means'for igniting said product's andlocated adjacent the upstream end of said"co'mbustion chamber, flameholder means for maintaining a flamefront including a plurality of arms rotatable about points intermediate their ends, said arms being rotated about separate axes which axes are substantially par'allel'to the fiuid'flow through said combustion chamher but' eccentrically located in said chamber, wake producing elements carried. by the extremities of said' arms'to form rotatable flameholder elements, means for synchronizing the relative rotative. angular position of said arms with respect to each other, and means for regulating the speed of rotation of said arms.

5. Inga duct forming an elongated combustion chamher, meansfcr. feeding'produ'cts' of combustion to said combustion chamber for burning therein, an exhaust H nozzle downstream from said combustion chamber and rec'eivingtheexhaust fiuids'therefrom, means for igniting said products and located adjacent the upstream end of said combustion chamber, flameholder means for maintaining a flamefront including a plurality of arms rotatable about points intermediate their ends, said arms being rotated about separate axes which axes, are substantially parallel. to the fluid flow through said combustion' chamber but eccentrically located in said chamber, wake producing elements carried by the extremities of said arms to form rotatable flameholder elements, means for synchronizing the relative rotative angular position of said arms with respect to each other comprising. mechanism for interconnecting said arms, and means for regulating the speed of rotation of said arms.

6. In a, duct forming a combustion chamber, means for feeding products of combustion to said combustion chamber for burning therein, an exhaust nozzle downstream from said combustion chamber and receiving the exhaust fluids therefrom, means for igniting said prodnets and locatedadjacent the upstream end of said com- 'bustion. chamber, flameholder means for maintaining a flamefront including a plurality of arms rotatable about points intermediate their ends, said arms being rotated about separate axes which are substantially parallel to the fluid, flow through said combustion chamber, said axis of rotation being off center in said duct, wake producing. elements carried by the extremities of said'arms to form rotatable flameholder elements, means for synchronizing the relative rotative angular position of said arms with respect to each other, power means for rotating said arms, and means for regulating said power means.

7. In atduct forming a combustion chamber, means for feeding products of combustion to said combustion chamber for burning therein, an exhaust nozzle downstream from said combustion chamber and receiving the and located adjacent the upstream end of said combustion chamber, flameholder means for maintaining a flamefront including a plurality of arms rotatable about points intermediate their ends, said arms being rotated about separate axes which are substantially parallel to the fluid flow through said combustion chamber, said axis of rotation being eccentrically located in said duct, wake producing elements carried by the extremities of said arms to form rotatable flameholder elements, said elements having their spanwise dimensions at a predetermined angle relative to the plane of rotation of said arms and relative to the axis of fluid flow whereby the velocity vector of the fluid is perpendicular to the spanwise edges of said elements.

8. In a duct forming a combustion chamber, means for feeding products of combustion to said combustion chamber for burning therein, an exhaust nozzle downstream from said combustion chamber and receiving the exhaust fluids therefrom, means for igniting said products and located adjacent the upstream end of said combustion chamber, flameholder means for maintaining a flamefront including a plurality of arms rotatable about points intermediate their ends, said arms being rotated about separate axes which are substantially parallel to the fluid flow through said combustion chamber but eccentrically located therein, wake producing elements carried by said arms to form rotatable flameholder elements, and means for rotating said arms at a speed where where v is the natural frequency of the first transverse mode of fluid oscillation and can be expressed where C=velocity of sound in the combustion chamber in ft. per second, D=combustion chamber diameter in ft.

9. In a duct forming a combustion chamber, means for feeding products of combustion to said combustion chamber for burningtherein, an exhaust nozzle downstream from said combustion chamber and receiving the exhaust fluids therefrom, means for igniting said prod nets and located adjacent the upstream end of said combustion chamber, flameholder means for maintaining a flamefront including a plurality of arms rotatable about points intermediate their ends, said arms being rotated about different axes, each of which is substantially parallel to the fluid flow through said combustion chamber,

V-shaped wake producing elements carried by the extremities of said arms to form rotatable flameholder elements, means for positioning said arms for rotation in a predetermined phase relationship and the phase relative angular positions of said arms ranging between 0 and and means for rotating said arms.

References Cited in the file of this patent UNITED STATES PATENTS 2,768,498 Karcher Oct. 30, 1956 2,784,551 Karlby et al. Mar. 12, 1957 FOREIGN PATENTS 713,863 Great Britain Aug. 18, 1954 

